The invention relates to a process and apparatus for separating isobutene from C4-hydrocarbon mixtures. The C4-fraction used in this process, which has in most cases already been substantially freed from butadiene by extractive distillation, generally contains principally 1-butene, 2-butene (cis or trans) and n-butane in addition to isobutene. These C4-hydrocarbon mixtures cannot be separated economically by simple fractional distillation owing to the inadequate boiling-point difference between its components. Higher-performance, more selective, physical and additionally also chemical separation processes are therefore used.
The latter are also used in particular for removing isobutene, since this differs from the remaining C4-components through, inter alia, its higher reactivity. The chemical separation process on which the invention is based involves reaction of isobutene with a primary C3- or C4-alkanol in the presence of a catalyst to give the corresponding tertiary ether. After distillative separation thereof from the remainder of the C4-hydrocarbon mixture followed by cleavage back into isobutene and the corresponding primary C3- or C4-alkanol, the isobutene is obtained as the top product after a further distillative separation step. At this point, some examples are defined which are used hereinafter:
The word xe2x80x9cinternalsxe2x80x9d is used as a general term for trays and packing. Such internals, which also enable the immobilization of heterogeneous catalysts (see, for example, EP-B1-0 396 650, EP-D1-0 008 860, are referred to more precisely below as reactive internals (reaction trays, reactive packing). Internals for exclusively distillative purposes are referred to as conventional.
In a known process for isolating isobutene from C4-hydrocarbon mixtures, isobutene present therein is reacted with primary C3- or C4-alkanol to give the corresponding tertiary ether (EP-E-0 015 513 and EP-B-0 003 305). The reaction is catalyzed by ion exchanger resins. In order to achieve an adequate conversion, the reaction is carried out in a cascade of three reactors, between which cooling is effected. In the first reactor, kinetic aspects mean that the etherification is carried out at a higher temperature than in the second and third reactors, in which the temperature is set so that it has a favorable effect on the chemical equilibrium of the exothermic etherification. In order to achieve satisfactory conversions, it is also necessary to use a significant excess of primary alkanol. If isobutanol is used, the molar starting ratio between the latter and isobutene is approximately 1.7:1 (ether formed: isobutyl tert-butyl ether (IBTBE)). The corresponding tertiary ether is then separated from the remainder of the C4-hydrocarbon mixture in a distillation column. After thermal, catalytic cleavage of the ether, isobutene is obtained as the top product in a further distillation.
It is an object of the present invention to improve the processes described above for isolating isobutene from C4-hydrocarbon mixtures so that
1. the complexity of the apparatus is reduced, and
2. higher conversions are achieved, so that the molar starting ratio between the primary C3- or C4-alkanol and isobutene can be reduced.
We have found that this object is achieved by a process for isolating isobutene from a hydrocarbon mixture by
a) combining the C4-hydrocarbon mixture with a primary C3- or C4-alkanol;
b) reacting the isobutene in the C4-hydrocarbon mixture with the primary C3- or C4-alkanol in the presence of a heterogeneous catalyst to give the corresponding tertiary ether of isobutene,
c) separating the resultant reaction mixture into the relatively low-boiling, unetherified C4-hydrocarbons and the relatively higher-boiling tertiary ether of isobutene with the aid of a distillation column, where the C4-hydrocarbons are taken off at the top, and the tertiary ether of isobutene obtained at the bottom is transferred into a reactor,
d) cleaving the ether into isobutene and the corresponding primary C3- or C4-alkanol,
e) distilling this mixture from d) in a further distillation column, and taking off the isobutene as the top product.
The novel process comprises carrying out step a) in a zone containing reactive internals and containing the catalyst for carrying out step b) arranged in such a way that the zone is integrated into a distillation column, that a reactive distillation takes place in this zone, and that the C3- or C4-alkanol is fed to the distillation column above the zone and the C4-hydrocarbon mixture is fed to the distillation column below the zone.
The invention also provides an apparatus for carrying out the abovementioned process, which apparatus has the following devices:
i) a distillation column containing reactive and conventional internals
ii) two phase separators,
iii) a catalyst bed,
iv) a distillation column, and
v) connecting lines between devices i) to iv).
The inventive idea of being able to carry out the basic chemical separation process with the aid of a reactive distillation is based on the knowledge that a reaction (isobutene reacted with a primary alkanol) and a distillation can be carried out under the same conditions. The prerequisite is that the product has a higher boiling point than the starting materials, that the starting materials have different boiling points (prerequisite for countercurrent flow of the starting materials), and that the boiling point difference between the two starting materials is not excessive (adequate residence time of the starting materials is necessary in the zone in which the reaction takes place). In addition, it must be possible to carry out the reaction with high conversion under the distillation conditions, the selectivity must be high, and the corresponding etherification must be reversible.
In the present process, a more precise term for the zone as etherification location would be reactive distillation zone. This is arranged in the central part of the distillation column and, in accordance with the invention, contains reactive internals, with pure distillations taking place in the regions above and below this zone. For these regions, conventional internals are provided. The reactive internals can be a catalyst on the corresponding residence-time trays or in their outflow or, in the case of packing, included therein in the manner of a pocket. Packing elements can be correspondingly coated. The catalyst used can be suitable known catalysts. These include, for example, ion exchanger resins in the [H+] form (Bayer Levatit(copyright)). Depending on the thermal stability of the catalysts, the reaction temperature and consequently the pressure can vary. The corresponding increase in reaction rate allows a more compact design of the column. The temperatures should be selected so that the catalyst is not damaged. At the same time, it is desirable to keep the condensation temperatures significantly above ambient temperature. The correlations between concentration, pressure and temperature for a given system are known per se.
For the isobutene/primary C3- or C4-alkanol/relevant tertiary ether system, a temperature damage limit for the catalyst of, for example, 150xc2x0 C. thus gives a possible pressure range of from 3 to 8 bar and temperatures of from 25 to 190xc2x0 C. in the interior of the column. A pressure of about 5 bar is preferred. The temperature in the reactive distillation zone of the column is then from 35xc2x0 C. to 190xc2x0 C., preferably from 50xc2x0 C. to 160xc2x0 C. Higher temperatures may not only be damaging to the catalyst, but also favor side-reactions, for example elimination of water from the alkanol, oligomerization of isobutene or etherification of two alkanols.
The dependence of the reaction rate on the system temperature determines the requisite catalyst volume. An essential advantage of the reactive distillation described, owing to the countercurrent flow of the starting materials, is the high etherification conversion caused by the more favorable location of the chemical equilibrium. For the same molar starting ratio between the primary alcohol and isobutene, higher conversions are achieved than in the process described by EP-B-0 015 513 or EP-B-0 003 305 (cascade of three reactors). On the other hand, the high conversion allows the molar starting ratio between the primary alcohol and isobutene to be significantly reduced. For the process described, the latter is preferably 1.1:1. A lower alkanol content in the reactive distillation zone reduces the amount of xe2x80x9cresidual alkanolxe2x80x9d to be removed from the still. This is associated firstly with a reduction in the circulation streams and secondly the chemical equilibrium of the ether cleavage is favorably affected. Owing to the high conversions, the process described is particularly suitable in etherification for the preparation of C4-hydrocarbon mixtures having a particularly low isobutene content (isobutene is harmful to certain processes). For this purpose, a higher starting material content of the primary alcohol is advantageously selected. The molar starting ratio between the primary alcohol and isobutene is generally from about 1.7:1 to 1.0:1.
Owing to the reduction in the apparatus complexity, achieved according to the invention by the integration of the reaction zone (in EP-B-0 015 513 and EP-B-0 003 305, a cascade of three reactors for the etherification) into the distillation column, not only are material costs reduced in plant construction, but the energy necessary for the process is also reduced. The heat of reaction from the exothermic etherification is advantageously also utilized directly for the distillation.